This invention relates generally to vertical carriers or boats for holding semiconductor wafers during heat processing and is particularly concerned with a vertical carrier designed to effectively support large semiconductor wafers having nominal diameters equal to or greater than about 200 millimeters, preferably wafers having nominal diameters of about 300 millimeters or greater.
Semiconductor wafers, especially those made of silicon, may be conventionally processed by placing them horizontally into a holding device or carrier at intervals in the vertical direction and exposing the wafers' surfaces to high temperature gases in a furnace, usually to form an oxide film on these surfaces or to deposit certain atoms therein. To maximize the amount of surface area exposed to the heat treatment, the wafers are usually held in "boats" or carriers typically comprised of parallel vertical supports or rails having relatively shallow slots or grooves evenly spaced along their length. The slots in one support are normally aligned with slots in the other support so a wafer can be jointly received by a corresponding slot in each support. By placing wafers in appropriate slots on the supports, the boat can carry a stack of wafers separated from each other so that each wafer is exposed to the heat treatment.
In the past, vertical boats and carriers have been designed to support wafers having nominal diameters of 200 millimeters or less. These size wafers are typically supported by teeth formed by slots or grooves on the vertical rails, which teeth extend inward from the edge of the wafer only a very short distance, usually less than about 20 millimeters. Unfortunately, when such design is utilized to support larger wafers, i.e., wafers having a nominal diameter greater than about 200 millimeters, the wafers are deflected by their own weight and tend to sag. As the temperature in the furnace rises, this sagging or deformation results in crystal dislocation or "slip" and other stresses on the wafer. Although "slip" typically begins to occur at about 1200.degree. C. for wafers having nominal diameters of 200 millimeters, it probably occurs at a temperature of 1000.degree. C. or less for wafers having diameters of 300 millimeters or larger. Crystal dislocations caused by stresses on the wafers result in a decrease in the number of chips that can be made on a wafer. This reduction in product yield results in increased manufacturing costs.
Various techniques have been suggested in an attempt to decrease the bending stress on larger wafers.
One method suggested is to locate the rails or vertical supports of the boat or carrier more toward the front of the carrier where the wafers are loaded. This, however, is difficult because of the need for an unobstructed wafer loading path. Another technique for decreasing bending stress on large wafers is taught in U.S. Pat. No. 5,492,229, the disclosure of which patent is incorporated herein by reference in its entirety. This patent teaches the use of relatively long support teeth, i.e., the support arms formed by long slits or slots on each support rail, with small contact pads located at or near the end of the teeth for supporting the wafers toward their center and not at their edges. According to this patent, the contact pads or support projections are located such that the inner portion of each wafer is supported by the pad while the peripheral portion, i.e., the portion of the wafer which extends from the edge of the wafer inward a distance of up to 10% of the wafer's radius, does not contact the pads or arms. By supporting the wafers at their inner portion with small pads, this design not only reduces the stress on the wafer caused by its own weight but also decreases heat stress caused by direct heat transfer to the wafer from the slits in the vertical supports.
The use of long support arms or teeth with small contact pads at their end to support wafers as proposed in the above-discussed patent has several disadvantages. First, the wafer support is far from uniform as it relies on one small contact pad located at or near the end of each tooth. Second, the strength of each long tooth is relatively weak to begin with and is further reduced when material is removed from the top of each tooth to form the small support pads. This, in turn, reduces the overall structural strength of the carrier. Finally, the manufacturing of rails with long support teeth is an expensive proposition that can greatly increase the costs of the wafer carrier.